With advancements in technology, antennas have changed in size and range. One specific category of antenna that may be used to provide two-way communication is the omnidirectional collinear array. These antennas typically consist of multiple radiators placed end-to-end and fed in phase.
FIG. 1 is a cross-sectional view of a collinear antenna 10 commonly used for two-way communication. The collinear antenna 10 has a differential transmission line 24 attached to a feed point 14 so as to excite a lower coaxial sleeve 16 and an upper radiator segment 18. A phasing inductor 20 and a series-appended radiator 22 extends from the upper radiator segment 18. The collinear antenna 10 may be described as, but not limited to, a traditional five-eighths-wave over half-wave series-fed collinear antenna. This collinear antenna configuration exhibits gain over a basic sleeve dipole, but also yields undesirable increases in driving resistance and element Q. These characteristics result in an impedance mismatch and a reduction in useful bandwidth.
In order to counter the resulting mismatch and restore efficient radio frequency-power transfer, it is common practice to implement a tuned impedance-matching network between the feed point and the coaxial feedline. Unfortunately, this addition introduces higher manufacturing cost, greater structural complexity, reduced operating bandwidth, and increased radio frequency losses.
Also, in order to faithfully replicate resonant microwave circuitry, antennas of this type may be wholly or partially constructed as a printed circuit board (PCB) based strip line structure. PCB construction offers the advantage of accurate high-volume replication, but the liabilities of constructing radio frequency networks and radiators on a PCB are also well known. Specifically, two-dimensional strip line sleeves generally yield inferior common-mode rejection when compared to a fully surrounding cylindrical sleeve. More significantly, virtually any PCB substrate material one might select will introduce greater dielectric loss than a structure constructed in the dielectric medium of air. The amount of loss is usually related inversely to price. When a PCB substrate material with high dissipation losses, such as FR4, is introduced for the purpose of minimizing antenna cost, losses will be relatively high and may prove unacceptable. Conversely, when a low-dissipation material is used to control losses, the cost may prove prohibitive.
Thus, a heretofore unaddressed need exists in the industry to consider and address the aforementioned deficiencies and inadequacies.